Dr. Ahmed Fathalla IbrahimAssociate Professor of AnatomyCollege of MedicineKing Saud UniversityE-mail: ahmedfathala@hotmail.com

Dr. Zeenat ZaidiAssociate Professor of AnatomyCollege of MedicineKing Saud Universityzeenatzaidi@hotmail.com

DEVELOPMENT OF SKELETAL &

MUSCULAR SYSTEM

OBJECTIVESAt the end of the lecture, the student should be able to describe

the:• Division of the intraembryonic mesoderm.• Formation and division of somite.• Derivatives of somite (sclerotome & myotome). • Formation of somatic and splanchnic layers of lateral plate

mesoderm and their derivatives.• Difference between the intramembranous and

intracartilaginous ossification and name the bones developed by these processes.

• The origin of skeletal, cardiac and smooth muscles in the body.

Intraembryonic Mesoderm Develops between

Ectoderm & Endoderm EXCEPT in the central axis of embryo where NOTOCHORD is found.

Differentiates into 3 parts:1. Paraxial mesoderm:

on each side of notochord.

2. Intermediate mesoderm

3. Lateral mesoderm

Notochord: stimulates neural tube formation

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Paraxial mesoderm divides into units (somites).

Lateral mesoderm divides by intraembryonic coelom into:Somatic mesoderm

(between ectoderm & coelom).

Splanchnic mesoderm (between endoderm & coelom).

Somatic mesoderm

Intraembryonic celome

Splanchnic mesoderm

• A small cavity, the myocele appears in each somite but soon disappears

• Each somite divides into ventromedial part called sclerotome and dorsolateral part the dermomyotome

Sclerotome: Bones of the axial skeleton (cranium, vertebral column, ribs and sternum) Myotome: Associated muscles of the back &Dermatome: The adjacent dermis of the skin

• Based on the mode of development, there are two types of bones in the body:

• Cartilage bones, that develop via intracartilagenous (endochondral ossification). A cartilage model first forms and is eventually replaced with bone e.g. formation of the bones of the axial & appendicular skeletons and the cranial base.

• Membrane bones, that develop via intramembranous ossification. Bone forms directly from mesenchymal cells without the prior formation of cartilage e.g. majority of bones of the face and skull

Development of the Bones

Development of Cranium (Skull)

The skull bones develops from mesoderm around the developing brain.

The skull consists of:1. Neurocranium: protective case for brain2. Viscerocranium: skeleton of face

Bones of skull ossify either by:• Intracartilagenous (Endochondral)

ossification, or• Intramembranous ossification

Bones of skull that ossify by intramembranous ossification:1. Frontal2. Parietal3. Zygomatic4. Squamous temporal5. Maxilla6. Mandible

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Base of skull develops by intracartilagenous

ossification

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Mesenchyme from lateral mesoderm

Induces growth of mesenchyme & its transformation into cartilage

Cartilage ossifies by:Endochondral ossification

Myoblasts migrate from myotomes to form:Muscles of limbs

Bone in cartilaginous state

Appearance of primary ossification centers: ossification of diaphysis

Appearance of secondary ossification centers: ossification of epiphysis

Ossification of epiphseal plate: Complete union of epiphysis & diaphysis

Diaphysis

Epiphysis

Epiphyseal plate of cartilage

BIRTH PUBERTY

Diaphysis

Bone increases in length by proliferation of epiphyseal plate

Growth of bone stops

Ossification of Long Bones

Bone age is a good index of general maturation. Bone age is determined by:1. Appearance of ossification centers in diaphysis & epiphysis (specific for each bone & sex)2. Disappearance of epiphyseal plate (specific for each bone & sex)

Epiphysis

Development of Joints

They develop from mesoderm lying between bones:

In fibrous joints: mesoderm differentiates into dense fibrous connective tissue.

In cartilaginous joints: mesoderm differentiates into cartilage.

In synovial joints: a synovial cavity is formed inside mesoderm; mesoderm differentiates into synovial membrane, capsule & ligaments.

Summary of Development of BoneAll bones develop from MESODERM. AXIAL SKELETON:

*Vertebrae, ribs & sternum: from sclerotomes of somites (paraxial mesoderm)*Skull: from mesoderm surrounding the brain

APPENDICULAR SKELETON: from somatic part of lateral mesoderm

All bones ossify by endochondral ossification EXCEPT:1. Some bones of skull2. Clavicle

12 weeks embryo

DEVELOPMENT OF MUSCLES All muscles develop from MESODERM EXCEPT muscles of iris

(eyeball) and myoepithelial cells of mammary & sweat glands which develop from ECTODERM

Cardiac muscles develop from: splanchnic part of lateral mesoderm

Smooth muscles: In the wall of viscera from: splanchnic part of lateral mesoderm In the wall of blood & lymphatic vessels from: somatic part of

lateral mesoderm All skeletal muscles develop from myotomes of paraxial

mesoderm EXCEPT some head & neck muscles which develop from mesoderm of pharyngeal arches

Myotome• Each myotome divides

into: Dorsal Epaxial division Ventral Hypaxial

division

• The Epaxial division gives rise to the muscles of the back (extensor muscles of the vertebral column)

• Ventral Hypaxial division gives rise to the muscles of the ventral body wall

Myoblasts migrate

into limb and giveLimb

muscles

QUESTION 1

Which one of the following group of muscles are derivatives from epaxial division of myotomes?

1. Muscles of back2. Muscles of limbs3. Muscles of viscera4. Cardiac muscles

QUESTION 2

Which one of the following bones ossifies by intramembranous ossification?

1. Vertebra2. Humerus3. Ribs4. Mandible

QUESTION 3Regarding the ossification of long bones, which

one of the following statement is correct?1. Primary ossification centre appears after birth.2. Secondary ossification centre leads into

ossification of diaphysis.3. Long bones ossify by intramembranous

ossification.4. When epiphysis unites with diaphysis, growth

of bone stops.

thank u & good luck